Glass substrates having excellent heat resistance and transparency, and a low coefficient of linear expansion are used as substrates for liquid crystal display elements, organic EL display elements, color filters, solar cells, and the like. Recently, plastic materials have attracted attention as an alternative to glass substrates for display devices to satisfy requirements, such as reduction in size, thickness and weight, excellent impact resistance, and flexibility.
Examples of materials recently used as such plastic substrates include polyesters such as PET (polyethylene terephthalate) or PEN (polyethylene naphthalate) and the like, polycarbonate, polyethersulfone, cyclo olefin resins, epoxy resins, acrylic resins, and the like. However, these materials can cause problems, such as product warpage, wire disconnection and the like, due to their considerably high coefficients of thermal expansion. In addition, a resin such as a polyamide resin exhibiting a low coefficient of thermal expansion can be used as the substrate. However, the polyamide resin is not suitable for the substrate material due to very low transparency, high birefringence, moisture absorption, and the like.
To solve such problems, a composite sheet having a very low thermal expansion and exhibiting good flexibility, heat resistance and transparency is prepared using glass fiber cloths and a low-anisotropy silicone rubber compound as a matrix. However, to use such a composite sheet as a substrate for displays, a barrier layer for preventing moisture permeability and gas permeation from outside should be introduced. However, such a barrier layer exhibits high elastic modulus, different mechanical properties from the matrix, and weak interfacial adhesion between two layers, thereby causing problems in terms of flexural resistance, flexibility and durability, such as cracking, and the like.